The present invention relates to an apparatus for measurement of biomagnetism which employs magnetic detection coils and superconducting quantum interference devices.
An apparatus for measurement of biomagnetism, which is used for magnetocardiography and magnetoencephalography, has so far employed a technique that a magnetic detection coil made of superconducting wires detects magnetic signals generated by an organism to transmit to a superconducting quantum interference device (hereinafter referred to as SQUID). A SQUID has a superconductor ring with a Josephson junction, in which a voltage between both ends of the Josephson junction cyclically varies with a period of Φ0=h/2e (Wb) according to a magnetic flux penetrating the SQUID.
It has been a typical technique in magnetocardiography and magnetoencephalography that a magnetic detection coil made of superconducting wires detects magnetic signals generated by a measurement object as a magnetic flux, which is transmitted to a SQUID. The magnetic detection coil helps to eliminate noise due to environmental magnetic fields to increase a Signal/Noise (S/N) ratio.
FIG. 17 is a schematic diagram illustrating architecture of a Flux Locked Loop (FLL) circuit in a typical apparatus for measurement of a magnetic field.
In an FLL circuit 1700, current produced by a magnetic flux penetrating a magnetic detection coil 1701 flows through the magnetic detection coil 1701 and an input coil 1702. Accordingly, the input coil 1702 creates a magnetic flux, which is transmitted to a SQUID 1703. The SQUID 1703, which has a superconducting ring with a Joesphson joint, is supplied a bias current by a bias current source 1705. A voltage between both ends of the SQUID 1703 cyclically varies with a period of Φ0=h/2e (Wb) according to a magnetic flux penetrating the SQUID 1703. In the FLL circuit 1700, a feedback circuit, which includes a preamp 1706, an integrator 1707, a feedback resistor 1708 and a feedback coil 1704, is provided in a rear stage of the SQUID 1703. The feedback coil 1704 feeds back a magnetic flux so as to cancel a change in the magnetic flux penetrating the SQUID 1703.
It is possible to obtain current flowing through the feedback coil 1704 by measuring a potential difference between both ends of the feedback resistor 1708. The magnetic flux penetrating the SQUID 1703 can be calculated based on this current value.
A circuit which has the architecture described above is called FLL circuit. The FLL circuit 1700 provides an output voltage proportional to the magnetic field detected by the magnetic detection coil 1701.
Description is given of a typical magnetic detection coil used for an apparatus for measurement of biomagnetism with reference to FIG. 18.
FIGS. 18A to 18E are schematic diagrams illustrating magnetic detection coils used for an apparatus for measurement of biomagnetism.
FIG. 18A shows a zero-order differential magnetic detection coil (magnetometer), FIG. 18B a first-order differential magnetic detection coil, FIG. 18C a second-order differential magnetic detection coil, FIG. 18D a zero-order differential magnetic detection coil formed on a thin film substrate and FIG. 18E a first-order differential magnetic detection coil formed on a thin film substrate.
As shown in these drawings, a magnetic detection coil typically employs architecture in which superconducting wires are wound around a cylindrical bobbin or the other architecture in which a thin film is formed on a substrate.
As shown in FIG. 18A, a zero-order differential magnetic detection coil 181 has a coil 181a which is made of a bobbin 1811 wound by one turn of superconducting wire. A magnetic flux ΦM in the following equation (1) detected by the zero-order differential magnetic detection coil 181 is represented by a magnetic flux Φ181a penetrating the coil 181a as follows:ΦM=Φ181a  (1)
Because the magnetic flux ΦM is equivalent to the magnetic flux Φ181a penetrating the coil 181a, the zero-order differential magnetic detection coil 181 is able to obtain magnetic signals greater than the first-order and second-order differential magnetic detection coils to be described later. However, the zero-order differential magnetic detection coil does not decrease an effect of environmental magnetic fields at all, directly experiencing noise due to the environmental magnetic fields. Accordingly, the zero-order differential magnetic detection coil 181 is usually used in a magnetically shielded room.
In this specification, a distance between centers of coils is referred to as “center-to-center distance”.
Given the magnetic flux ΦM shown in FIG. 18A is a positive magnetic signal, current flows through the zero-order differential magnetic detection coil 181 in a direction of a fine arrow, which is drawn along the coil. Hereinafter, a signal representative of a magnetic flux pointing upward is defined as a positive magnetic signal. A direction of current, which flows through a coil when a positive magnetic signal is detected, is represented by a fine arrow.
As shown in FIG. 18B, a first-order differential magnetic detection coil 182 has coils 182a and 182b. The coil 182a has one turn of superconducting wire, which is wound around a bobbin 1821 in a first direction. The coil 182b has one turn of superconducting wires which is wound around the bobbin 1821 in a second direction opposite to the first direction, lying a predetermined distance vertically apart from the coil 182a. A magnetic flux ΦG1 in the following equation (2) detected by the first-order differential magnetic detection coil 182 is represented by a magnetic flux Φ182a penetrating the coil 182a and a magnetic flux Φ182b penetrating the coil 182b as follows:ΦG1=Φ182a−Φ182b  (2)
The reason why the magnetic flux Φ182b has a minus value is that the coil 182b is wound in the opposite direction.
It should be noted that taking a difference is referred to as “differentiating”, taking a first-order difference as “first-order differentiating” and taking a second-order difference as “second-order differentiating” in this specification.
The coil 182a is located proximity to a detection object and the coil 182b is located relatively away from it. Because environmental magnetic fields of spatial uniformity are cancelled, it is possible to detect only a magnetic flux deriving from the detection object.
It should be noted that a vertical direction is meant to represent a direction perpendicular to a plane including a coil and a horizontal direction is meant to represent a direction in parallel with this plane. In this connection, it may be possible to allow the vertical direction to coincide with a direction of measurement of a magnetic field.
As shown in FIG. 18C, a second-order differential magnetic detection coil 183 has coils 183a, 183b and 183c. The coil 183a has one turn of superconducting wire wound around a bobbin 1831 in a first direction. The coil 183b has two turns of superconducting wire wound around the bobbin 1831 in a second direction opposite to the first direction, lying a predetermined distance vertically apart from the coil 183a. The coil 183c has one turn of superconducting wire wound around the bobbin 1831 in the first direction, lying a predetermined distance vertically apart from the coil 183b. A magnetic flux ΦG2 in the following equation (3) detected by the second-order differential magnetic detection coil 183 is represented by a magnetic flux Φ183a penetrating the coil 183a, a magnetic flux Φ183b penetrating the coil 183b and a magnetic flux Φ183c penetrating the coil 183c as follows:ΦG2=Φ183a−2Φ183b+Φ183c  (3)
As described above, the second-order differential magnetic detection coil 183 differentiates magnetic fluxes in two steps in a vertical direction, thereby decreasing an effect due to both environmental magnetic fields of spatial uniformity and environmental magnetic fields having a first-order gradient. As a result, the second-order differential magnetic detection coil 183 is able to decrease the effect of the environmental magnetic fields more than the first-order differential magnetic detection coil 182 which is only able to decrease the effect of the environmental magnetic fields of spatial uniformity. When a magnetic signal is included in the magnetic flux Φ183b penetrating the coil 183b and the magnetic flux Φ183c penetrating the coil 183c, a magnetic signal detected by the second-order differential magnetic detection coil 183 will decrease. It is understood that the higher order a differential magnetic detection coil possesses, the less effect of environmental magnetic fields will exist. However, a detected magnetic signal will decrease accordingly. In this way, a tradeoff study is necessary to solve the irreconcilable situations described above. A magnetometer, a first-order differential magnetic detection coil, or a second-order differential magnetic detection coil has been so far typically used for biomagnetism in conjunction with a magnetically shielded room according to magnitude of environmental magnetic fields.
There is a technique to use a superconducting thin film instead of superconducting wires for a magnetic detection coil. FIG. 18D shows a zero-order differential magnetic detection coil 184, in which a superconducting thin film is formed on a substrate 1841. A magnetic flux Φ184a detected by a coil 184a is transmitted to a SQUID 1842 formed on the same substrate 1841. FIG. 18E shows a first-order differential magnetic detection coil 185, in which coils 185a and 185b having directions opposite to each other are formed on a substrate 1851. A difference Φ185a−Φ185b between a magnetic flux Φ185a detected by the coil 185a and a magnetic flux Φ185b detected by the coil 185b is transmitted to a SQUID 1852 formed on the same substrate 1851. An advantage of using the superconducting thin film is that it is possible to determine and materialize an accurate area of a magnetic detection coil.
Several types of arrangements for magnetic detection coils have been proposed taking into account characteristic features of the differential coils described above. As an example, a technique has been proposed, in which plural types of magnetic detection coils having different differential orders are placed at a measurement point so as to calculate and estimate magnetic field sources or a distribution of the magnetic field sources in an organism. Patent document No. 1: Japanese Published Patent Application 09-084777 (claim 1, paragraph 0015, FIG. 1).
However, as shown in FIGS. 18A to 18C, there have been limited arrangements in which only a magnetic field differentiated in a certain direction is detected. These arrangements have a problem that when environmental magnetic fields are strong because a magnetically shielded room is not available, for example, it is not possible to adequately reduce an effect of them. One possible technique is to increase order for a differential magnetic detection coil so as to decrease this effect. Although the effect can be decreased by this technique, it is inevitably accompanied by a reduction in a magnetic signal to be detected.
A magnetic detection coil using a superconducting thin film has a problem that it is intrinsically difficult to form a coil three-dimensionally.
In addition, it appears to be unproductive to combine these two types of coils, because magnetic detection coils with three-dimensional structure as shown in FIGS. 18A to 18C and magnetic detection coils formed on a superconducting thin film as shown in FIGS. 18D and 18E are different from each other in terms of usage, structure and manufacturing processes.